Make wxSharedPtr::Release() really MT-safe.

Make wxSharedPtr::Release() really MT-safe.

Using wxAtomicDec() is not enough, its result also must be checked as it will
return 0 only in one of the threads if multiple threads call it in parallel,
while the old test for m_count==0 could pass for more than one thread,
resulting in deleting the same pointer more than once.

Closes #15227.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74065 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775